1. Field of the Invention
The invention relates to an electric power supply unit for providing a predetermined output voltage obtained by converting the power supply voltage of a given dc power supply such as a battery, and to a portable device equipped with such power supply unit.
2. Description of the Related Art
A power supply unit such as a series regulator converts the power supply voltage of a dc power supply to a predetermined output voltage. This type of power unit must be capable of stably supplying the output voltage to a load without being affected by the fluctuations in the load and/or the power supply voltage. Therefore, in order to improve the transient response of the power supply unit in the process of controlling its output voltage, the power supply unit is mostly provided with a voltage control section, which, however, consumes much current.
If a power supply unit, equipped with such a voltage control section that consumes a large current as stated above, is applied to a portable device, e.g. a mobile telephone, adapted to operate not only in an active mode (or heavy load mode) but also in a sleep mode (or light load mode, e.g. standby mode), the amount of wasteful current consumed in the voltage control section in the sleep mode is not negligibly small.
To circumvent this problem, the power supply unit may be provided with a high-speed stable voltage supply section having an operational amplifier that consumes a large current, and a low-speed stable voltage supply section having an operational amplifier that consumes a small current, as disclosed in Japanese Patent Application Laid Open No. 2001-117650 (referred to as Patent Document 1). The output terminals of these operational amplifiers are connected to the gate of a common large-current output transistor via switching means such that the high-speed stable voltage supply section is exclusively enabled in the active mode, while the low-speed stable voltage supply section is exclusively enabled in the sleep mode. In the event of switching the operational mode of the power supply unit from one mode to the other, a common time interval is set up in which both the high-speed stable voltage supply section and the low-speed stable-voltage supply section are simultaneously turned on, thereby controlling the amount of the current consumed in the enabled stable-voltage supply section, according to the condition of the load.
However, it is necessary for the power supply unit of the Patent Document 1 to acquire an external mode signal for switching the operational mode, but some of portable devices powered by a portable power supply unit are not supported for such mode signal. In those cases, desired switching of the mode cannot be obtained. In addition, the prior art power supply unit requires an external controller for controlling the mode signal in accordance with the condition of the load.
A further problem exists in the prior art power supply unit in that both the high-speed stable voltage supply section and low-speed stable voltage supply section are controlled by a common output transistor and hence that the unit has a poor transient response when the common output transistor is controlled by the slow stable-voltage supply section.